Ever since the existence of cosmic rays with individual energies of several Joules was established in the 1960s, speculation has raged as to whether cosmic particles of mean energy of 2 Joules are created in our Milky Way or in distant extragalactic objects.

In a paper that was published in Science on 22 September 2017, the Pierre Auger Collaboration reported observations demonstrating that cosmic rays with energies a million times greater than that of the protons accelerated in the Large Hadron Collider come from much further away than from our own Galaxy.

Earlier this year the physics magazine physicworld.com reported about that paper, which was now chosen to be included in the “Physics World Top Ten Breakthroughs of 2017”. The Physics World Top Ten Breakthroughs of 2017 are awarded to research reported in physicsworld.com in 2017. The Top Ten are chosen by Physics World editors from a shortlist that is drawn-up based on popularity with our readers. The selection criteria are